1. Field of the Invention
This invention relates to a color image sensing device and more particularly to an image sensing device which is capable of automatically carrying out white balance adjustments.
2. Description of the Prior Art
A color image sensing device loses white balance when the color temperature of the light source varies. To overcome this problem, the image of a white or colorless photographic image is first sensed and, for example, a red color signal (R), a green color signal (G) and a blue color signal (B) which are obtained are adjusted to equal ratios. This process is called white balance adjustment. Generally, the white balance adjustment is accomplished by adjusting amplification gains of at least two or three different kinds of monochrome signals or two kinds of color difference signals obtained from the three kinds of monochrome signals. Since it is troublesome for users to carry out this adjustment manually, proposals have been made for various types of image sensing devices which are arranged to automatically accomplish the white balance adjustment.
An example of a prior art color image sensing device having an automatic white balance adjusting function is shown in FIG. 1 of the accompanying drawings. In FIG. 1, the prior art device includes an optical system 1; an image sensor 2 which recieves light passing through the optical system 1, senses the image of an object to be photographed which is formed thereon, and converts it to an electrical signal; and a chrominance signal separation circuit 3 which divides the electrical signal obtained from the image sensor 2 into three monochrome signals R, G and B. The monochrome signals R and B are respectively supplied to process circuits 6 and 8 via gain control circuits 4 and 5. The monochrome signal G is supplied directly to a process circuit 7. At the process circuits 6, 7 and 8, the signals R, G and B are subjected to gamma correction, aperture equalization and clamping processes as necessary. After these process circuits 6, 7 and 8, the respective signals R, G and B are supplied to a matrix circuit 9. The signals R, G and B are matrix processed to obtain a luminance signal (hereinafter called signal Y), a color difference signal R-Y and another color difference signal B-Y. The signals Y, R-Y and B-Y are processed at an encoder circuit 10 into a television signal. The television signal is produced from an output terminal 11.
The white balance adjustment is accomplished in the following manner: A perfectly white or colorless object is photographed. Then, the signals R, G and B thus obtained are supplied via the process circuits 6, 7 an 8 respective to average detection circuits 12, 13 and 14. The average detection circuits 12, 13 and 14 then perform integral detection and produce outputs corresponding to the respective intensities of these monochrome signals. The outputs of the average detection circuits 12 and 13 are compared with each other at a comparator 15. The comparator 15 then produces an output which corresponds to the difference in intensity between the monochrome signals R and G. If a switch 17 is on, the output of the comparator 15 controls a voltage storing circuit 19. For example, if the average value of the signal R is higher than that of the signal G, the voltage stored at the voltage storing circuit 19 is decreased to the gain control circuit lowers the amplification of the signal R.
This closed loop of a negative feedback circuit is formed while the switch 17 is on. The negative feedback loop becomes stable when the levels of the signals R and G are equalized. Upon stabilization of the negative feedback loop, the switch 17 is turned off. With the switch 17 turned off, a voltage which determines the amplification gain of the gain control circuit 4 required for the stabilization is stored at the voltage storing circuit 19. Thus, after the switch 17 is turned off the amplification gain of the gain control circuit 4 is fixed according to the voltage stored at the voltage storing circuit 19.
Another negative feedback loop which includes the gain control circuit 5 for the signal B operates in exactly the same manner. More specifically, the signal B which has undergone the processing of the process circuit 8 is supplied to an average detection circuit 14. A comparator 16 compares the intensity of the signal B with that of the signal G. Then, if the switch 18 is on, the feedback stabilizes the loop in the manner mentioned in the foregoing until the levels of the signals B and G are equalized. Upon completion of the control, the switch 18 is turned off. The amplification of the gain control circuit 5 is then controlled by the voltage stored at a voltage storing circuit 20. The levels of all the signals R, G and B can now be equalized by simultaneously turning on both the switches 17 and 18. With white balance adjustment accomplished in this manner, the voltages which determine the amplification gains of the gain control circuits 4 and 5 at that instant are stored at the voltage storing circuits 19 and 20. After that, for an ordinary photographic operation, the amplification gains of the gain control circuits 4 and 5 are controlled with the voltages stored at the voltage storing circuits 19 and 20, to permit photography under a satisfactory white balance condition. The length of time required for keeping the switches 17 and 18 on for carrying out the white balance adjustment must be arranged to be at least a period of time required for one field portion of a picture signal.
In the aforementioned color image sensing device, use of a white or colorless object is indispensable. For perfect white balance, the white or colorless object to be used must be such that it covers the whole image plane. In many cases however, such a large white or colorless object is not readily available.
A recently proposed image sensing device has a white and transmissive plate or the like inserted in a light path during white balance adjustment. This white balance adjusting arrangement obviates the need to use any white or colorless object to be photographed for that purpose. This method, however, requires an additional mechanical arrangement, which inevitably results in a more complex, larger device.